Many vehicles are used over a wide range of drive conditions, including both forward and reverse movement. The powertrain systems, in particular internal-combustion engines, however, have desirable operating conditions, including engine speed range, where they run most efficiently. Consequently, automotive transmissions capable of efficiently transmitting power at a variety of speed ratios are frequently employed. Transmission speed ratio is the ratio of input shaft speed to output shaft speed. When the vehicle is at low speed, the transmission is usually operated at a high speed ratio such that it multiplies the engine torque for improved acceleration. At high vehicle speed, operating the transmission at a low speed ratio permits an engine speed associated with quiet, fuel efficient cruising.
A common type of automatic transmission includes a gearbox capable of alternately establishing a fixed number of power flow paths, each associated with a fixed speed ratio. The gearbox includes a number of shift elements such as wet clutches and brakes, where their frictional interfaces are continually lubricated with automatic transmission fluid. A particular power flow path is established by engaging a particular subset of the shift elements. To shift from one power flow path to another power flow path with a different speed ratio, one or more shift elements must be released while one or more other shift elements must be engaged. Some shift elements are passive devices such as one way clutches, while other shift elements engage or disengage in response to commands from a controller. For example, in many automatic transmissions, the shift devices are hydraulically controlled friction elements such as wet clutches or brakes. The controller regulates the torque capacity of the shift element by regulating an electrical current to a solenoid, which adjusts a force on a valve which, in turn, adjusts a pressure in a hydraulic circuit.
A modern automatic transmission is controlled by a microprocessor which adjusts the torque capacity of each wet shift element, including any lock-up clutch, at regular intervals. At each interval, the controller gathers information indicating the driver's intent, such as the positions of the shifter (PRNDL), the accelerator pedal, and the brake pedal. The controller also gathers information about the current operating state of the vehicle, such as speed, and of the engine. Increasingly, information is also available from other sources, such as anti-lock brake controllers and GPS systems. Using this information, the controller determines whether to maintain the currently established power flow path or to shift to a different power flow path. If the controller decides to shift to a different power flow path, the controller then adjusts the torque capacities of the off-going shift elements and the on-coming shift elements in a coordinated manner in order to make the transition as smooth as possible. However, it remains a challenge to accurately deliver desired clutch torque for both engagement and release processes. This is because the wet clutch hydrodynamically transmits torque by means of viscous shear across fluid film between rotating clutch plates with or without mechanical asperity contact at the frictional interfaces. This hydrodynamic torque is particularly sensitive to fluid conditions at the interface. Specifically, the amount of hydrodynamic torque is affected by the change rate of oil film thickness and slip speed during clutch engagement and exhibits highly non-linear behaviors with respect to actuator force profile and slip speed, making it difficult for the controller to consistently deliver desired torque under all shift conditions.
A wet clutch bench tester is widely utilized in order to improve clutch design features during a transmission development process. The industry-standard clutch test stand, which is often referred to as SAE No. 2 tester, is an inertia-absorption-type brake machine, typically equipped with a pneumatic actuator with limited control authority. It is utilized for evaluating clutch performance stability and durability during engagement duty cycles, but not capable of recreating realistic clutch slip and actuator force profiles for the purpose of shift control development. There are other clutch testers with advanced features such as enhanced electrical motor control and programmable hydraulic actuator, enabling the use of the methodology patented in U.S. Pat. No. 6,923,049 for accurately replicating clutch slip and actuator force profiles during torque phase and inertia phase of shifting as observed in a vehicle. However, clutch torque measurements obtained from such advanced testers do not correlate well with those observed in a vehicle, even if slip and force profiles are accurately replicated. There is a need to invent and establish a clutch bench test methodology that enables accurate replication of clutch engagement torque behaviors, as observed in a vehicle, to support transmission shift control development.